In the mobile communication system, a quality of a wireless line in a downlink (base station→mobile station) largely depends on a thermal noise and interference waves. Hence, there arises a big problem of how the interference waves are restrained.
The related arts involve restraining the interference waves exerted on a certain focused area and therefore manually adjusting a down tilt of an antenna and a physical direction of a sector antenna or controlling transmission power.
FIGS. 1A and 1B illustrate a pre-adaptation example and a post-adaptation example of the down tilt of the sector antenna. As illustrated in FIG. 1A, it is assumed that radio beams from a sector antenna c of a base station 5A exert the large interference with a certain focused area AR1. In this case, as illustrated in FIG. 1B, the down tilt of the sector antenna c is adjusted so that the radio beams from the sector antenna c do not reach the focused area AR1. With this adjustment, the interference may be restrained.
FIGS. 2A and 2B illustrate a pre-adaptation example and a post-adaptation example of the directional adjustments of the sector antennae. As illustrated in FIG. 2A, before adjusting the directions of the sector antennae, it is assumed that the radio beams from a sector antenna a (direction of 90°) reach the focused area AR1 to thereby exert the large influence of the interference on the focused area AR1. In this case, as illustrated in FIG. 2B, the antenna directions of the sector antennae a and b are adjusted so that the radio beams from the sector antennae a and b do not reach the focused area AR1. This adjustment enables the interference to be restrained.
FIGS. 3A and 3B illustrate a pre-adaptation example and a post-adaptation example of transmission power control. As illustrated in FIG. 3A, in the case of transmitting the radio waves with high-output transmission power toward each of the mobile stations 1A, 1B and 1C from the sector antenna c of the base station 5A, an assumption is that the interference largely affects the focused area AR1. In this case, as illustrated in FIG. 3B, the transmission power from the sector antenna c is controlled, in which the transmission power control is executed so that the radio waves are transmitted with the higher transmission power as the mobile station is distanced farther from the base station 5A. With this contrivance, the interference with the focused area AR1 can be restrained.
Patent document 1: Japanese Laid-Open Patent Publication No. 2006-115291
Patent document 2: Japanese Laid-Open Patent Publication No. 2000-295650
Patent document 3: International Publication No. WO2005/025253
The technique illustrated in FIGS. 1A and 1B involves increasing the number of mobile terminals deviating from the wireless area due to the narrowed wireless area formed (covered) by the base station.
Further, the technique illustrated in FIGS. 2A and 2B is disabled, if a zone, from which is wished to avoid the interference, exists in the sector direction of 30 degrees in the state of FIG. 2B, from reducing the interference with this zone.
Moreover, the technique illustrated in FIGS. 3A and 3B, since a level of a signal addressed to the mobile station 1C in FIG. 3B is comparatively high due to a far position of the mobile station 1C, may not restrain the occurrence of the interference caused therefrom.